This article describes the process of deleting a domain from an Exchange account.
In order to manage domains you need to login to CONTROL PANEL and navigate to Services > Domains.
On the next screen select the Remove Domain checkbox and click on Save Changes.
Note: It is recommended to remove custom domains from the account prior termination, otherwise you will not be able to re-add the domain manually within 30 days. To reset the propagation time call support.
For more information on adding domains have a look at the article: How Do I Add A Domain To My Exchange Account?